The preparation of pharmaceutical agents used to treat human disease is enabled by advances in catalytic science. This project has the long-term goal of developing a broad class of inexpensive nonmetal catalysts that promote atom transfer and bond activation processes via formal oxidation state cycling in much the same way that late transition metal catalysts operate. Within this overarching goal, the primary focus of this proposal is the design and evaluation of phosphorus-based catalysts that function in the PIII?PV redox couple. While phosphines are well-established in catalysis as spectator ligands for transition metal catalysis and as nucleophilic catalysts, this research will investigate phosphorus-based catalysts of novel composition and structure that explore the structural and electronic conditions required to enable new catalytically-relevant reactivity via reversible PII?PV oxidation state cycling. The first major effort is the development of phosphine-catalyzed O-atom transfer methods that result in reductive difunctionalization of carbonyl compounds. The second major effort is the development of amine functionalization reactions that are initiated by phosphorus-mediated bond activation. The proposed research is expected to yield new practical catalytic methods for the construction of pharmacologically-relevant small molecules that meet the challenges of sustainable synthesis, and an improved fundamental understanding the interplay between structure and reactivity in the p-block that will underpin future development of nonmetals for atom transfer, bond activation, and catalysis. Taken together, these outcomes will advance a new and powerful modality in nonmetal-based catalysis.